In general, communication systems antennas emit and/or receive communication signals propagating through air and space. Numerous different types of communications antennas are in use today. Antennas transmit (and receive) electromagnetic waves made of a combination of electric and magnetic fields propagating in a certain direction. The electric and magnetic fields are perpendicular to each other and are perpendicular to the direction of propagation of the electromagnetic wave (EM wave).
The orientation of the electric and magnetic fields with respect to the surface of the earth determines whether the electromagnetic wave (the communication signal) is vertically or horizontally polarized. If the electric field is parallel to the earth, the EM wave is horizontally polarized. If the electric field is perpendicular to the earth, the EM wave is vertically polarized. The structure and orientation of an antenna dictates whether the antenna emits or receives vertically or horizontally polarized EM waves (some structures emit and/or receive circularly polarized EM waves, however circular polarization will not be discussed herein). Generally, both the transmit and receive antennas in a communications system must be of the same polarization for proper transmission and reception.
Antennas radiate and receive energy in many different directions, however, most antennas radiate or receive energy in a very specific geometric radiation pattern that is non-uniform over a 360 degree circle parallel to the earth's surface. Antennas exhibiting this characteristic are called directional antennas. Some antennas are constructed (or oriented) to radiate or receive energy in all directions parallel to the surface of the earth. These antennas are called omni-directional antennas.
For example, a half-wave dipole antenna has a radiation pattern in the shape of a doughnut. Most of the energy radiated from a half-wave dipole is radiated substantially from right angles to the length of the dipole. As such, almost no energy is radiated along the lines extending along the length of the dipole. A half-wave dipole mounted horizontally to the earth (horizontal polarization) is a directional antenna (i.e. minimal radiation in the directions along the length of the dipole). A half-wave dipole mounted vertically to the earth (vertical polarization), therefore, is an omni-directional antenna (i.e. equal amount of radiation in all directions parallel to the earth). In the transmission mode, a dipole antenna should be pointed broadside to the desired direction of transmission or, in the reception mode, pointed broadside to the point of transmission of the signal from a transmitter.
Standard ground mobile cellular communications systems (ground-to-ground) use vertically polarized signals in the 800-900 MHz range. In order to reuse the same RF spectrum as ground-to-ground cellular systems, aircraft cellular communications system (air-to-ground) use horizontally polarized signals to prevent interference with the vertically polarized ground mobile cellular communication systems. While RF power management control techniques may help reduce some of this interference, a substantial amount of interference is still present. As such, the design of the aircraft antenna (coupled with the attributes of the operating environment, i.e., air-to-ground communication from a moving aircraft) plays a critical role in the performance of the aircraft cellular communications system. It must provide a high rejection of the vertically polarized ground communications signals.
One type of antenna that may be used in an aircraft cellular communications system is a stacked horizontal dipole antenna. It provides near omni-directional patterns with horizontal polarization. This type of antenna is usually oriented lengthwise along the centerline of the aircraft, and for that reason, an undesirable null in the antenna pattern exists along the axis of the antenna, fore and aft of the aircraft. Loss of the communication link is possible if the aircraft is turning or maintains an inbound/outbound flight profile with the ground station. Further, radiation of a vertical polarized component also increases for increasing angles off antenna boresight and the amount of horizontal component to vertical component suppression degrades overall performance. Consequently, use of a stacked horizontal dipole antenna results in a high potential for interference and/or loss of communications.
Accordingly, there exists a need for an antenna for use with aircraft cellular communication systems that transmits and receives horizontally polarized signals and provides high rejection of the vertically polarized ground mobile cellular communications signals. Further, the antenna should be capable of minimizing any potential for interference with ground mobile and fixed cell sites and reducing susceptibility to interference from sites that transmit vertical polarized signals. Also needed is an antenna that provides a deep null in the radiation pattern to prevent interference from ground mobile and fixed cell sites positioned substantially directly below the aircraft in flight. Additionally, there is needed an omni-directional antenna to provide maximum radiation outward to the horizon so that the communications link is independent of aircraft flight profile and ground station location.